Suppose we had this sentence:
Hannibal Romanos vicisset, nisi plurimi elephantes periissent.
Now let's put this in indirect speech:
*Credo Hannibalem vicisse.
We want to say, "I believe that Hannibal would have won". But the above means something else: "I believe that Hannibal won". How should the irrealis be rendered in indirect speech?
I have already looked it up, and the below is what Allen and Greenough have to say (which matches what I seemed to remember). In short, use the future participle with fuisse:
Credo Hannibalem victurum fuisse.
[*] b. In changing a Condition contrary to fact (§ 517) into the Indirect Discourse, the following points require notice:—
The Protasis always remains unchanged in tense.
The Apodosis, if active, takes a peculiar infinitive form, made by combining the Participle in -ūrus with fuisse .
If the verb of the Apodosis is passive or has no supine stem, the periphrasis futūrum fuisse ut (with the Imperfect Subjunctive) must be used.
An Indicative in the Apodosis becomes a Perfect Infinitive.
Examples are:—
“nec se superstitem filiae futurum fuisse, nisi spem ulciscendae mortis êius in auxilio commilitonum habuisset ” (Liv. 3.50.7) , and that he should not now be a survivor, etc., unless he had had hope, etc. [Direct: non superstes essem , nisi habuissem .]
“illud Asia cogitet, nullam a se neque belli externi neque discordiarum domesticarum calamitatem afuturam fuisse, si hoc imperio non teneretur ” (Q. Fr. 1.1.34) , let Asia (personified) think of this, that no disaster, etc., would not be hers, if she were not held by this government. [Direct: abesset , si non tenerer .]
“quid inimicitiarum creditis [me] excepturum fuisse, si insontis lacessissem ” (Q. C. 6.10.18) , what enmities do you think I should have incurred, if I had wantonly assailed the innocent? [excepissem ... si lacessissem.]
“invitum se dicere, nec dicturum fuisse, ni caritas rei publicae vinceret ” (Liv. 2.2) , that he spoke unwillingly and should not have spoken, did not love for the state prevail. [Direct: nec dixissem ... ni vinceret .]
“nisi eo tempore quidam nuntii de Caesaris victoria ... essent allati, existimabant plerique futurum fuisse uti [oppidum] amitteretur ” (B. C. 3.101) , most people thought that unless at that time reports of Cæsar's victory had been brought, the town would have been lost. [Direct: nisi essent allati ... amissum esset .]
“quorum si aetas potuisset esse longinquior, futurum fuisse ut omnibus perfectis artibus hominum vita erudiretur ” (Tusc. 3.69) , if life could have been longer, human existence would have been embellished by every art in its perfection. [Direct: si potuisset ... erudita esset .]
“ at plerique existimant, si acrius insequi voluisset, bellum eo die potuisse finire ” (B. C. 3.51) , but most people think that, if he had chosen to follow up the pursuit more vigorously, he could have ended the war on that day. [Direct: si voluisset ... potuit .]
“Caesar respondit ... si alicûius iniuriae sibi conscius fuisset, non fuisse difficile cavere ” (B. G. 1.14) , Cæsar replied that if [the Roman people] had been aware of any wrong act, it would not have been hard for them to take precautions. [Direct: si fuisset , non difficile fuit (§ 517. c).]
[*] Note 1.--In Indirect Discourse Present Conditions contrary to fact are not distinguished in the apodosis from Past Conditions contrary to fact, but the protasis may keep them distinct.
[*] Note 2.--The periphrasis futurum fuisse ut is sometimes used from choice when there is no necessity for resorting to it, but not in Cæsar or Cicero.
[*] Note 3.--Very rarely the Future Infinitive is used in the Indirect Discourse to express the Apodosis of a Present Condition contrary to fact. Only four or five examples of this use occur in classic authors: as,Titurius clamabat si Caesar adesset neque Carnutes, etc., “neque Eburones tanta cum contemptione nostra ad castra venturos esse” (B. G. 5.29) , Titurius cried out that if Cæsar were present, neither would the Carnutes, etc., nor would the Eburones be coming to our camp with such contempt, [Direct: si adesset ... venirent .]